1. Field of the Invention
This invention relates generally to Satellite Positioning Systems (“SPS”) devices, and in particular to a wireless cellular device capable of utilizing cell information with the SPS derived data to locate a wireless cellular device.
2. Related Art
The worldwide utilization of wireless devices such as two-way radios, portable televisions, personal communication system (“PCS”), personal digital assistants (“PDAs”) cellular telephones (also known a “mobile phones”), Bluetooth, satellite radio receivers and Satellite Positioning Systems (“SPS”) such as Global Positioning Systems (“GPS”), also known as NAVSTAR, is growing at a rapid pace. As the number of people employing wireless devices increases, the number of features offered by wireless service providers also increases, as does the  integration of these wireless devices in other products.
The number of features offered by wireless service providers is increasingly matching the features offered by traditional land-line telephone service providers. Features such as call waiting, call forwarding, caller identification (“caller I.D.”), three-way calling, data transmission and others are commonly offered by both land-line and wireless service providers. These features generally operate in the same manner on both wireless devices and land-line telephones. “Enhanced 911” (also known as E911) services, however, operate differently on wireless devices than a 911 service call (normally referred to as a “911” call) does on land-line telephones.
When a 911 call is placed from a land-line telephone, the 911 reception center receives the call and determines the origin of the call. In case the caller fails, or forgets, to identify his or her location, the 911 reception center is able to obtain the location from which the call was made from the land-line telephone switching network and send emergency personnel to the location of the call.
If instead, an E911 call is placed from a wireless device such as a cellular telephone, the E911 reception center receives the call but cannot determine the origin of the call. If the caller fails, or forgets, to identify his or her location, the E911 reception center is unable to obtain the location of the call because the mobile switching network is different than the land-line telephone switching  network. At present, the best that the E911 reception center may possibly do is determine the location of the basestation corresponding to the cell from which the call was placed. Unfortunately, typical cells in a cellular network system may cover an area with approximately a 30 mile diameter.
However, the United States Congress, through the Federal Communications Commission (FCC), has enacted a requirement that cellular telephones be locatable to within 50 feet once an emergency call, such as an E911, is placed by a given cellular telephone. This type of position data would assist police, paramedics, and other law enforcement and public service personnel, as well as other agencies that may need to have legal rights to determine the position of specific cellular telephone. Therefore, there is a need for a system that determines the location of a wireless device within 50 feet once an emergency call such as an E911 is placed by a given wireless device such as a cellular telephone.
Further, SPS data that is supplied to the mobile telephone may be utilized by the mobile telephone user for directions, location of other locations that the mobile telephone user is attempting to locate, determination of relative location of the mobile telephone user to other landmarks, directions for the mobile telephone user via internet maps or other SPS mapping techniques, etc. Such data may be of utilized for other application in addition to the E911 service, and would be very useful for cellular and PCS subscribers. 
A proposed solution to this problem has been to utilize a wireless positioning system that includes satellites and/or pseudolites (such as basestations) to triangulate the position of a wireless device such as a cellular telephone. GPS is an example of a SPS that may be utilized by a wireless device in combination with an appropriate GPS receiver to pinpoint the location of the wireless device on earth. The array of GPS satellites transmits highly accurate, time coded information that permits a receiver to calculate its exact location in terms of latitude and longitude on earth as well as the altitude above sea level. The GPS system is designed to provide a base navigation system with accuracy to within 100 meters for non-military use and greater precision for the military (with Selective Availability ON).
The space segment of the GPS system is a constellation of satellites orbiting above the earth that contain transmitters, which send highly accurate timing information to GPS receivers on earth. The fully implemented GPS system consists of 21 main operational satellites plus three active spare satellites. These satellites are arranged in six orbits, each orbit containing three or four satellites. The orbital planes form a 55° angle with the equator. The satellites orbit at a height of 10,898 nautical miles (20,200 kilometers) above earth with orbital periods for each satellite of approximately 12 hours.
Each of the orbiting satellites contains four highly accurate atomic clocks.  These provide precision timing pulses used to generate a unique binary code (also known as a pseudo random or pseudo noise “PN” code) that is transmitted to earth. The PN code identifies the specific satellite in the constellation. The satellite also transmits a set of digitally coded ephemeris data that completely defines the precise orbit of the satellite. The ephemeris data indicates where the satellite is at any given time, and its location may be specified in terms of the satellite ground track in precise latitude and longitude measurements. The information in the ephemeris data is coded and transmitted from the satellite providing an accurate indication of the exact position of the satellite above the earth at any given time. A ground control station updates the ephemeris data of the satellite once per day to ensure accuracy.
A GPS receiver configured in a wireless device is designed to pick up signals from three, four, or more satellites simultaneously. The GPS receiver decodes the information and, utilizing the time and ephemeris data, calculates the approximate position of the wireless device. The GPS receiver contains a floating-point processor that performs the necessary calculations and may output a decimal display of latitude and longitude as well as altitude on the handset. Readings from three satellites are necessary for latitude and longitude information. A fourth satellite reading is required in order to compute altitude.
These techniques, however, still do not perform well in dense environments  where the location of a wireless device (such as a cellular telephone) is usually hindered in dense environments such as downtown city blocks. A SPS system within the wireless device should have the capability to acquire and track the SPS satellites under the conditions that the typical user of a wireless device will encounter. Some of these conditions include utilization of the wireless device indoors and in dense urban areas that have a limited sky view, such as in downtown areas with skyscrapers blocking the views of the normally available satellites, etc. While these environments are typically manageable for terrestrial-based wireless communications systems, they are difficult environments for a SPS system to operate. For example, traditional “autonomous mode” SPS systems (i.e., SPS systems where the SPS receiver acquires the signals from the SPS satellites, tracks the satellites, and, if desired, performs navigation without any outside information being delivered to the SPS system) have problems with long Time To First Fix (“TTFF”) times and, additionally, have a limited ability to acquire the SPS satellite signals under indoor or limited sky-view conditions.
Even with some additional information, TTFF times may be over thirty seconds because the ephemeris data must be acquired from the SPS system itself, and the SPS receiver typically needs a strong signal to acquire the ephemeris data reliably. These characteristics of a SPS system typically impact the reliability of position availability and power consumption in wireless devices. Typically, the  accuracy of location-based solutions may vary from 150 meter to 300 meter in these types of environments. As a result, locating a wireless device in a 300 meter radius zone is unlikely unless there are other methods to help narrow the search.
Attempts at solving this problem have included utilizing pseudolites (such as basestations in a cellular telephone network) in combination with SPS, such as GPS, to determine the location of the wireless device. As an example, U.S. Pat. No. 5,874,914, entitle “GPS Receiver Utilizing a Communication Link,” issued to Norman F. Krasner on Feb. 23, 1999, which is herein incorporated by reference, describes a method where a basestation (also known as the Mobile Telephone Switching Office “MTSO”) transmits SPS satellite information, including Doppler information, to a remote unit utilizing a cellular data link, and computing pseudoranges to the in-view satellites without receiving or utilizing satellite ephemeris information.
Additionally, U.S. Pat. No. 6,208,290, entitled “GPS Receiver Utilizing a Communication Link,” issued to Norman F. Krasner on Mar. 27, 2001, which is herein incorporated by reference, describers a method for deriving the approximate location of SPS receiver from a cellular communication system information source. The actual surveyed latitude and longitude of the cell site is then utilized to compute the approximate Doppler. This may cause errors (especially altitude) since many cellular towers (associated with cellular basestations) are positioned at  raised grounds for better coverage and if their altitude is provided for aiding then there may be an error in the location computation.
Unfortunately, these types of combined pseudolite and SPS approaches typically require that the wireless device is always receiving aiding information from the cellular communication system even in non-dense environments where the wireless device may easily determine its position via SPS alone. This results in inefficient power consumption and even slower acquisition times when SPS data is easily obtainable. Also, the aiding information is based on the position of a cell tower (i.e., basestation) that may cause errors as described above. Therefore, there is a need for a system that determines the location of a wireless device within 50 feet in a dense environment with selective aiding from non-SPS pseudolites.